1. Field of the Invention
The present invention relates to an ultrasonic flaw detection method and apparatus for detecting defect portions in weld portions of coarse grained materials (i.e. in weld portions of austenitic steel), and particularly to an ultrasonic flaw detection method an apparatus that enable echoes from a defect portion to be detected at a high S/N (Signal to Noise) ratio.
2. Description of the Related Art
One of the nondestructive inspection methods used to nondestructively inspect machinery and structures and the like in various types of industrial plants, to detect deterioration in structural materials and defects such as blowholes in welds and flaws caused by damage, to evaluate the soundness of the machinery and structures and the like, and to predict the remaining lifespan thereof is the ultrasonic flaw detection method.
In a conventional ultrasonic flaw detection method, a defect is detected when transverse ultrasonic waves are propagated through a steel structure that is being inspected, and defect echoes generated when the ultrasonic waves collide with a defect and are reflected are detected, thus providing information on the defect. An example of an ultrasonic flaw detection apparatus for use in a TOFD (Time Of Flight Diffraction) type of detection method, which is known as an excellent ultrasonic flaw detection method for obtaining sizing information as it enables information such as the position and size of a defect to be obtained, is schematically shown in FIGS. 6A to 6C. A pair formed by an ultrasonic wave generator probe 2 and an ultrasonic wave receiver probe 3 that are both connected an ultrasonic transceiver 1 are positioned in the area where a nondestructive inspection is to be performed, for example, on both sides of a weld portion (a weld bead) 4a welded in a steel material 4 such that the two probes are substantially the same distance from the weld portion 4a. Ultrasonic flaw detection echoes propagated by the ultrasonic transceiver 1 through the steel material 4 via the ultrasonic wave generator probe 2 are received by the ultrasonic transceiver 1 via the ultrasonic wave receiver probe 3.
As is shown in FIG. 6C, contained in these received ultrasonic flaw detection echoes are surface transmission waves 5 that pass from the ultrasonic wave generator probe 2 through surface portions of the steel material 4 and arrive directly at the ultrasonic wave receiver probe 3, and bottom surface reflection waves 6 that are irradiated from the ultrasonic wave generator probe 2 into the steel material 4, arrive at the bottom surface of the steel material 4 and are then reflected, and then arrive at the ultrasonic wave receiver probe 3. If there is a defect portion 7 inside the weld portion 4a on a flat plane transversing the weld portion 4a and running in a straight line between the two probes 2 and 3, a portion of the ultrasonic waves irradiated into the steel material 4 are scattered by the distal end of the defect portion 7. They are then refracted resulting in defect portion echoes (defect scatter waves) 8 that arrive at the ultrasonic wave detector 3 later than the surface transmission waves 5 and earlier than the bottom surface reflection waves 6 being contained in the ultrasonic flaw detection echoes.
Moreover, as is shown in FIG. 6A, the received ultrasonic flaw detection echoes are processed by an image processing apparatus 9 that is connected to the ultrasonic transceiver 1 while moving the two probes 2 and 3 in parallel along the weld portion 4a. An image is then displayed, for example, as is shown in FIG. 6B, by plotting XY coordinates on a monitor screen 10 taking the length of time that lapses after the transmission of ultrasonic waves from the ultrasonic wave generator probe 2 as the X axis and the amount of movement of the probes 2 and 3 as the Y axis. In this case, when the probes 2 and 3 arrive at the transverse position of the defect portion 7, defect portion echoes 8 are detected. Accordingly, by corresponding the plotted Y coordinates of the defect portion echoes 8 to the amount of movement of the probes 2 and 3, information on the position of the defect portion 7 in the direction of movement of the probes 2 and 3 can be obtained. As the probes 2 and 3 continue their movement and pass the transverse position of the defect portion 7, the defect portion echoes 8 are no longer detected. Thus, information relating to the size of the defect portion 7 in the direction of movement of the probes 2 and 3 is obtained from the amount of the movement of the probes 2 and 3 while the defect portion echoes 8 are being detected, namely, is obtained from the plotted length of the defect portion echoes 8 in the Y axis direction. Furthermore, position information relating to the depth of the defect portion 7 is obtained from the length of time that lapses after the transmission of ultrasonic waves from the ultrasonic wave generator probe 2 until the defect portion echo 8 is detected, namely, is obtained from the plotted X coordinates of the defect portion echo 8 and from the rate of ultrasonic wave propagation through the steel material 4 that has been determined in advance.
When, for example, an ultrasonic flaw detection method is used to evaluate the soundness of weld portions in coarse grained materials (austenitic steel) such as 9% nickel steel and inconel and austenitic stainless steel that are widely used in atomic plants and chemical plants, there is a sizable attenuation in the transverse ultrasonic waves and flaw detection is difficult. Therefore, ultrasonic flaw detection using longitudinal ultrasonic waves is becoming more common.
However, because columnar crystals often appear in weld portions in austenitic steel, noise echoes from the columnar crystals are often generated even when the above longitudinal ultrasonic waves are used. Moreover, because the sizes of these noise echoes are substantially the same as the sizes of the defect portion echoes generated when a defect is present in the weld portion, it is not possible to distinguish between defect portion echoes and noise echoes by simple threshold value processing. Namely, in ultrasonic flaw detection methods for coarse grained materials, the problems of a low S/N ratio in defect portion echoes and a low flaw detection performance arise.
It is an aim of the present invention to provide an ultrasonic flaw detection method and apparatus that enable defect portion echoes to be detected with a high S/N ratio from among ultrasonic flaw detection echoes from weld portions in austenitic steel.
During repeated research into ways of improving S/N ratios in ultrasonic flaw detection performed on weld portions in austenitic steel, the present inventors noticed that because the noise echoes are reflection waves from grain boundaries and are formed by waves from countless reflection sources mutually interfering with each other, if waveforms obtained as ultrasonic flaw detection echoes are separated into the necessary frequency components (frequency bands), the waveform phase (the peak emergence position relative to the time axis) is different for each frequency band. The present inventors also noticed that, in contrast to this, in defect portion echoes, the waveforms all have the same phase even if the frequency bands are different, namely, the positions of peak emergence relative to the time axis all match. As a result, the present inventors discovered that by separating original waveforms of ultrasonic flaw detection echoes into the necessary frequency bands and then detecting peaks that have matching phases even though the frequency bands are different, it is possible to extract and thus detect only the peaks of defect portions, and thus the present inventors achieved the present invention.
Namely, in the ultrasonic flaw detection method and apparatus of the present invention: wideband longitudinal ultrasonic waves are irradiated from an ultrasonic wave generator probe onto a weld portion of a coarse grained material; from the waveforms of flaw detection echoes that are subsequently obtained, the highest frequency component that can be extracted using time frequency analysis is then extracted; subsequent xc2xd magnification frequency components are then extracted sequentially; waveforms of a necessary plurality of frequency bands from among each of the frequency bands that were extracted and have undergone waveform separation are then multiplied, and waveform peaks that are formed by the multiplication are detected as being defect portion echoes generated by defect portions in the coarse grained material weld portion; and, as a result, information on the defect portion is obtained from the detected defect portion echo.
Because the noise echoes are reflection waves from grain boundaries and are formed by waves from countless reflection sources mutually interfering with each other, if the highest frequency component that can be extracted using time frequency is extrated from the waveform of ultrasonic flaw detection echoes from coarse gained material weld portions and then subsaquent xc2xd magnification frequency components are extracted sequetially, then the waveform phase (the peak emergence position relative to the time axis) is different for each frequency band. In contrast to this, in the TOFD flaw detection method, scatterd waves from the tip end of the defect portion which are propagated as spherical waves are detected, and therefore, in defect portion echoes, the waveforms all have the same phase even if the frequency bands are different, and the positions of peak emergence relative to the time axis all match. As a result, if waveforms of the necessary frequency band components that have been extrated from the waveforms of ultrasonic flaw detection echoes and have undergone waveform seperation are multiplied, then while the noise echoes, whose phases do not match, are close to zero, defect portion echoes, whose phases do match, are amplified and form peaks. Accordingly, by detecting these peaks, the defect portion echoes can be extracted at a high S/N ratio.
Thus, according to the ultrasonic flaw detection method and apparatus of the present invention, the excellent effect of the accurate performing of ultrasonic flaw detection becoming possible is obtained.
Furthermore, in the ultrasonic flaw detection method and apparatus of the present invention: an ultrasonic wave generator probe for generating wideband longitudinal ultrasonic waves and an ultrasonic wave receiver probe are disposed symmetrically at positions on both sides of the coarse grained material weld portion; wideband longitudinal ultrasonic waves are irradiated from the ultrasonic wave generator probe onto the coarse grained material weld portion; from waveforms of flaw detection echoes that are subsequently obtained, the highest frequency component that can be extracted using time frequency analysis is then extracted; subsequent xc2xd magnification frequency components are then extracted sequentially; waveforms of a necessary plurality of frequency bands from among each of the frequency bands that were extracted and have undergone waveform separation are then multiplied, and waveform peaks that are formed by the multiplication are detected as being defect portion echoes generated by defect portions in the coarse grained material weld portion; and position information and size information about the defect portion are obtained by displaying the detected defect portion echoes as an image by plotting them on XY coordinates that take an amount of movement of the probes as one axis and a length of time lapsed from a transmission of an ultrasonic wave by the ultrasonic wave generator probe as another axis.
In this case, it is possible obtain accurate position information about a defect portion in the direction of movement of the probes from the amount the probes move after the detection of the defect portion echo commences based on a defect echo portion detected at a high S/N ratio when noise echoes have been separated and removed. In addition, accurate information about the size of the defect portion in the direction of movement of the probes can be obtained from the amount the probes move from the start of the detection of the defect portion echoes until the end of the detection thereof. Furthermore, accurate position information about the depth of the defect portion can be obtained based on the time the defect portion echoes are detected and the transmission speed of ultrasonic waves inside the coarse grained material, which is determined in advance.
The result of this is that the accuracy of detecting defect portions is improved. In cases, particularly, when this ultrasonic flaw detection method and apparatus are used in non-destructive inspections of an existing plant, the remaining lifespan diagnosis of the plant can be made with a high degree of accuracy.